This invention relates generally to parabolic trough reflectors and associated support structure; more specifically, this invention relates to flexible rectangular sheets that are supported along two opposing edges and thereby elastically deformed into parabolic troughs through the action of the distributed gravitational forces arising from their own weight.
Parabolic trough reflectors are used in solar energy conversion systems to collect and concentrate sunlight onto receiver elements. In order to accomplish this function, a parabolic trough reflector must track the sun; that is, it must pivot, roll, rotate, tilt, or otherwise move in some fashion so that its extended optical plane (plane containing its vertex line and its focal line) continuously bisects the solar disc as it moves across the sky.
There are two different kinds of solar energy conversion systems that use parabolic trough reflectors. Their distinguishing features relate primarily to the type of motion used to accomplish sun tracking.
One kind of system has parallel rows of parabolic trough reflectors whose longitudinal axes are permanently oriented in a fixed direction, usually in either a north-south direction or an east-west direction. In these systems, each reflector tilts (rolls) about an axis parallel to its focal line in order to track the sun. Systems of this kind are referred to herein as Tilting Trough Reflector (TTR) systems. TTR systems are by far the most common type of solar concentrating system currently in use, but they have three serious problems. The first problem with TTR systems is that, since the reflectors tilt in order to track the sun, the individual rows of reflectors must be separated in order to avoid mutual shadowing at low sun angles. This leads to poor land area utilization—which is less than 30% in many currently operating TTR systems. The second problem with TTR systems is that, during sun-tracking operations, the parabolic trough reflectors are subjected to directionally-varying forces and reflector support structures are exposed to severe bending and torsional stresses. Both the reflectors and the support hardware must be designed to function effectively in spite of these challenging mechanical requirements, and this significantly increases TTR system cost and decreases system reliability. The third problem with TTR systems is that, since the parabolic trough reflectors are permanently oriented in a fixed direction, they are exposed to severe wind-induced stresses whenever strong winds blow crosswise to the direction of their longitudinal axes. This places additional demands on both the reflectors and the support structure, again adding to hardware manufacturing costs and system complexity.
A second kind of parabolic trough reflector system avoids the inherent problems associated with TTR systems. U.S. Pat. No. 4,159,629 discloses a type of solar energy conversion system wherein the rows of parabolic trough reflectors do not have a fixed north-south or east-west orientation. For these systems, sun tracking is accomplished by rotating all the rows of reflectors and the associated rows of receiver elements as a single interconnected unit, with the rotation of the unit occurring in a horizontal plane about a central vertical axis. The individual reflectors do not tilt as they track the sun and the optical plane of each reflector is always held in a fixed vertical alignment. Since the reflectors do not tilt, they can be positioned side by side, thus providing efficient utilization of available land area. Also, the non-tilting parabolic trough reflectors provide directionally invariant loads to the reflectors and the reflector support hardware. This characteristic is very important because the design of the reflectors and the associated support structures can be greatly simplified—and their fabrication and installation costs can be significantly reduced. Finally, since the orientation of the rows of reflectors is not fixed in one specific direction, sun tracking for these systems may be temporarily interrupted whenever wind velocities reach a high level. The reflectors can then be reoriented so their longitudinal axes are aligned with wind direction. This feature greatly reduces the likelihood that wind-induced stresses could cause damage.
Solar energy conversion systems that use non-tilting parabolic trough reflectors will be referred to herein as Vertical Axis Trough Reflector (VATR) systems. (This designation is derived from the fact that sun tracking is accomplished by rotating the interconnected rows of trough reflectors about a central vertical axis.) Even though the VATR concept was originally proposed nearly 40 years ago, systems utilizing its obvious advantages have never been commercially deployed. This is primarily because much of the previously developed technology relating to parabolic trough reflectors cannot be effectively adapted to VATR systems.
The objective of this invention is to provide innovative designs for parabolic trough reflectors that can be used in VATR systems. The reflectors and support structures revealed herein can be fabricated by using simple manufacturing processes and the resulting hardware can be quickly and efficiently assembled in the field. These design/fabrication/assembly techniques will provide high-quality, low-cost parabolic trough reflectors and simple, dependable support structures. The reflectors and support structures will facilitate commercial deployment of solar energy conversion systems that make full use of the inherent advantages of the VATR concept.